The invention relates to a method of eliminating interference in a microphone signal.
Such methods are becoming more and more important for the voice input of commands and/or for hands-free telephones. In particular, they are used to correct the situation inside a motor vehicle.
A special situation frequently occurs in motor vehicles where a playback device, e.g., a radio, a tape player or a CD player, creates a noisy environment via a loudspeaker. This noise is superimposed as interference signal on a voice signal, picked up by a microphone, e.g., for the voice recognition or telephone transmission. In order to detect a voice input in a voice detector or to have an intelligible voice transmission via telephone, the microphone signal must be freed of as many interference signal components as possible.
The interference signal originating from an interference source, in particular a loudspeaker, not only travels directly, meaning via the shortest path, to the microphone, but appears also in the microphone signal via numerous reflections, as a superimposition of a plurality of echoes with different transit times. The total effect of the interference signal from the interference source to the microphone signal can be described by an a priori unknown transfer function of the space, e.g., the passenger space in a motor vehicle. This transfer function changes in dependence on the number of passengers in the vehicle and the position of the individual passengers. By simulating this transfer function and using this simulation to filter a reference signal from the interference source, a compensation signal can be generated, which supplies, for example, a pure voice signal that is free of any interference by subtracting it from the microphone signal. In the real case, the aforementioned simulation represents a more or less good approximation to the unknown transfer function, and the interference cannot be eliminated completely.
It is the object of the present invention to provide a method of eliminating interference in a microphone signal, which method displays good properties for eliminating interference along with an acceptable signal processing expenditure.